As long as you don't bend the lawn mower blades by hitting rocks or something else then you shouldn't have to replace the lawn mower blades that often.
Most times you can just sharpen the lawn mower blades by using a bench grinder or angle grinder and then use a blade balance tool to make sure the blades are balanced so they won't vibrate the lawn mower.
Every couple of months or once every season the lawn mower blades should be sharpened to keep the mower cutting the grass correctly.
If the blades have a large nick or is bent then you should replace the blades.
You should get at least 10 mowing seasons out of a set of blades before you should need to replace them.